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Chennai Metro Rail Ltd (CMRL) has invited bids for the construction of twin tunnels and five underground stations between Nathamuni and Kolathur. The rocky soil conditions make this one of the toughest and challenging stretches in phase-2 of the project. The 5.8 km stretch is the only underground section in corridor-5 from Sholinganallur to Madhavaram. This is the 11th tender floated for the construction of 118.9 km phase-2. Construction for the phase is expected to begin by mid-2021 with CMRL likely to start building the elevated line between Poonamallee and Porur bypass. The phase is expected to be ready by 2026 The firm that bags the contract will have to build twin tunnels for the upline and downline and connect them with five underground stations across at least two major localities Kolathur and Villivakkam. The stations will come up at Villivakkam bus terminus, Kolathur, Villivakkam suburban station, Srinivasa Nagar, and Nathamuni. The contractor will build entry-exit points of the stations, architectural finishes, ventilation shafts, and more. The deadline for the work is about four years. CMRL identified the line from Kolathur to Nathamuni as one of the two toughest stretches in phase-2 due to its soil conditions, primarily hard rocks like black granite or a mix of quartz charnockite. The condition could pose an engineering challenge as the tunnel boring machines will have to be operated at an appropriate speed - even as slow as below 8 m a day, to keep vibrations it may cause to a minimum, to prevent buildings above from being affected. Tunnels in this stretch will be at a depth of 15 m to 21 m. In phase-1, rock soil conditions slowed down the project. While a contractor abandoned work on the Anna Salai stretch claiming they hit rocky soil that was not anticipated, it took nearly six months for engineers to drill a 500 m rocky stretch near Central station. This also required engineers to replace the cutters or blades frequently. For details of CMRL tender for the tunnelling work, click here. Also read: Chennai Metro creates 3D model for phase 2